The phenomenon through which some flowering plant species are unable to successfully reproduce through self-pollination has been termed ‘self-incompatibility’ (SI). A definition of SI was accepted as proposed by Lundqvist, being ‘the inability of a fertile hermaphrodite seed-plant to produce zygotes after self-pollination’. This phraseology was in order to distinguish between SI and the effect of post-fertilisation barriers. SI has been described in about 30% of all flowering plants, and is the most important system for prevention of self-fertilization. The key part of this system is self-recognition, in which the pistil discriminates between self- or non-self-pollen to either inhibit or permit pollen tube germination and/or elongation and resulting fertilisation.
There are multiple genetic mechanisms that regulate and enable the system, SI does not represent a unique system. The molecular basis of the plant SI mechanism has been well studied in several groups of dicotyledonous plant species. Self-incompatibility locus (S locus) genes were identified in winged tobacco (Nicotiana elate) and Brassica rapa L. (syn. campestris) in the late 1980s.
Subsequent biological investigations using multiple approaches have identified SI factors, to help elucidate the molecular basis of SI mechanisms in these species.
Plant species within the Poaceae (grass and cereal) family, can display an obligate outbreeding reproductive habit controlled by a two-locus (termed S and Z) gametophytic SI system, in which the pollen genotype is autonomously controlled by its own genetic constitution. This system is conserved between allogamous Poaceae species, such as wild barley (Hordeum bulbosum L.) and cereal rye (Secale cereale L.) and has been found to be widely conserved within the family, but is expected to be genetically and mechanistically distinct from the well-characterised single-locus SI mechanisms of dicotyledonous plants.
The Poaceae-specific mechanism prevents self-fertilisation through arrest of self-generated gamete pollen tube elongation at the stigmatic surface. Although several of the key molecular signals involved in dicot-specific SI systems have been identified, the molecular basis of the Poaceae system remains unknown.
Free calcium concentrations are essential for directed cell growth in pollen tubes in many species. The role calcium plays was initially identified as increasing cell wall rigidity and regulating permeability. However, the specific concentration of calcium in the cell is critical, as free calcium is typically kept at c. 100 nm due to cellular metabolism being based on free phosphates, and if free cytosolic calcium levels elevate over this concentration, interference with the energy status of the cell will result due to the formation of calcium salts. Studies of calcium gradients within the pollen tube have identified an increased gradient at the active growing tip, and the increase has been postulated to be absorbed by the cellular growth.
Previously, calcium has been identified to play a role in regulation of the SI system in the Poaceae family. Treatment with calcium channel blockers (lanthanum and verapamil) has been demonstrated to inhibit the perennial ryegrass SI mechanism.
By treating excised stigmas with the chemical blocking agents self pollen was able to germinate.
The S and Z loci of perennial ryegrass (Lolium perenne L.) have been assigned to linkage groups (LGs) 1 and 2 respectively, in regions of known macrosynteny with the genomes of the inbreeding cereal species rice (Oryza sativa L.) and wheat (Triticum aestivum L.). Fine-structure mapping of the Poaceae SI loci was performed for blue canary grass (Phalaris coerulescens L.) and cereal rye (Secale cereale L.), and the candidate gene-containing regions were delimited to 0.26 cM and 1.5 cM intervals for the S and Z loci, respectively. The presence of gene-associated (cDNA-based) markers in these studies permitted comparative analysis to define map colinearity around the SI loci for related self-incompatible and self-compatible Poaceae species. The proposed 1.5 cM Z-containing region exhibited microsynteny with a BAC clone (OSJNBa0070011: GenBank Acc. No. AL606445) from rice chromosome 4 c. 125 kb in length, to which 12 predicted genes have been assigned.
For outbreeding crops, understanding and regulation of SI mechanisms can simplify and accelerate breeding procedures. For example, knowledge of the Solanaceae S-RNase-based SI system informed a method for almond cultivar development through use of a previously-characterised self-compatible (Sc) mutant line. The program involved introduction of the Sc allele into existing almond varieties for enhanced fixation of genes for favourable oil content and fatty acid composition. Inbreeding also enables simpler maintenance of agronomically elite lines.
In the standard semi-hybrid breeding scheme for outbreeding pasture crop species, two cultivar groups are intercrossed to generate a progeny population with increased yield owing to heterosis. However, around half the progeny are derived from an intracross within each parental group and do not receive the benefit of heterosis. Heterosis in forage mass has been also reported in perennial ryegrass and other grass crop species using the standard semi-hybrid breeding system.
In a new scheme with SI genotyping technology, an SI-allele restricted population is established, and intercrossing between the restricted population and a cultivar group generates a progeny population with a higher ratio of hybrid progeny. An experiment with red clover proved higher hybrid ratios in progenies and improved seed yields when the restricted populations were used.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.
It is an object of the present invention to overcome, or at least alleviate, one or more of the difficulties or deficiencies associated with the prior art.